80 research outputs found
Pulsation of EE Cam
EE Cam is a previously little studied Delta Scuti pulsator with amplitudes
between those of the HADS (High-Amplitude Delta Scuti stars) group and the
average low-amplitude pulsators. Since the size of stellar rotation determines
both which pulsation modes are selected by the star as well as their
amplitudes, the star offers a great opportunity to examine the astrophysical
connections. Extensive photometric measurements covering several months were
carried out. 15 significant pulsation frequencies were extracted. The dominant
mode at 4.934 cd was identified as a radial mode by examining the phase
shifts at different wavelengths. Medium-dispersion spectra yielded a
value of km s. This shows that EE Cam belongs to the
important transition region between the HADS and normal Delta Scuti stars.Comment: 13 pages, 3 figures, 3 table
A heuristic derivation of the uncertainty of the frequency determination in time series data
Context: Several approaches to estimate frequency, phase and amplitude errors
in time series analyses were reported in the literature, but they are either
time consuming to compute, grossly overestimating the error, or are based on
empirically determined criteria. Aims: A simple, but realistic estimate of the
frequency uncertainty in time series analyses. Methods: Synthetic data sets
with mono- and multi-periodic harmonic signals and with randomly distributed
amplitude, frequency and phase were generated and white noise added. We tried
to recover the input parameters with classical Fourier techniques and
investigated the error as a function of the relative level of noise, signal and
frequency difference. Results: We present simple formulas for the upper limit
of the amplitude, frequency and phase uncertainties in time-series analyses. We
also demonstrate the possibility to detect frequencies which are separated by
less than the classical frequency resolution and that the realistic frequency
error is at least 4 times smaller than the classical frequency resolution
Unevenly-sampled signals: a general formalism of the Lomb-Scargle periodogram
The periodogram is a popular tool that tests whether a signal consists only
of noise or if it also includes other components. The main issue of this method
is to define a critical detection threshold that allows identification of a
component other than noise, when a peak in the periodogram exceeds it. In the
case of signals sampled on a regular time grid, determination of such a
threshold is relatively simple. When the sampling is uneven, however, things
are more complicated. The most popular solution in this case is to use the
"Lomb-Scargle" periodogram, but this method can be used only when the noise is
the realization of a zero-mean, white (i.e. flat-spectrum) random process. In
this paper, we present a general formalism based on matrix algebra, which
permits analysis of the statistical properties of a periodogram independently
of the characteristics of noise (e.g. colored and/or non-stationary), as well
as the characteristics of sampling.Comment: 10 pages, 11 figures, Astronomy and Astrophysics, in pres
Cinderella - Comparison of INDEpendent RELative Least-squares Amplitudes
The identification of increasingly smaller signal from objects observed with
a non-perfect instrument in a noisy environment poses a challenge for a
statistically clean data analysis. We want to compute the probability of
frequencies determined in various data sets to be related or not, which cannot
be answered with a simple comparison of amplitudes. Our method provides a
statistical estimator for a given signal with different strengths in a set of
observations to be of instrumental origin or to be intrinsic. Based on the
spectral significance as an unbiased statistical quantity in frequency
analysis, Discrete Fourier Transforms (DFTs) of target and background light
curves are comparatively examined. The individual False-Alarm Probabilities are
used to deduce conditional probabilities for a peak in a target spectrum to be
real in spite of a corresponding peak in the spectrum of a background or of
comparison stars. Alternatively, we can compute joint probabilities of
frequencies to occur in the DFT spectra of several data sets simultaneously but
with different amplitude, which leads to composed spectral significances. These
are useful to investigate a star observed in different filters or during
several observing runs. The composed spectral significance is a measure for the
probability that none of coinciding peaks in the DFT spectra under
consideration are due to noise. Cinderella is a mathematical approach to a
general statistical problem. Its potential reaches beyond photometry from
ground or space: to all cases where a quantitative statistical comparison of
periodicities in different data sets is desired. Examples for the composed and
the conditional Cinderella mode for different observation setups are presented.Comment: 8 pages, 6 figures, A&A, in pres
Search for pulsating PMS stars in NGC 6383
A search for pulsating pre-main sequence (PMS) stars was performed in the
young open cluster NGC 6383 using CCD time series photometry in Johnson B & V
filters. With an age of only ~1.7 million years all cluster members later than
spectral type A0 have not reached the ZAMS yet, hence being ideal candidates
for investigating PMS pulsation among A and F type stars. In total 286 stars
have been analyzed using classical Fourier techniques. From about a dozen of
stars within the boundaries of the classical instability strip, two stars were
found to pulsate: NGC 6383 #170, with five frequencies simultaneously, and NGC
6383 #198, with a single frequency. In addition, NGC 6383 #152 is a suspected
PMS variable star, but our data remain inconclusive. Linear, non-adiabatic
models assuming PMS evolutionary phase and purely radial pulsation were
calculated for the two new PMS pulsators. NGC 6383 #170 appears to pulsate
radially in third and fifth overtones, while the other three frequencies seem
to be of non-radial nature. NGC 6383 #198 pulsates monoperiodically, most
probably in the third radial overtone. Magnitudes and B-V colours were
available in the literature for only one third of all stars and we used them
for calibrating the remaining.Comment: 12 pages, 11 figures, accepted by MNRA
MOST photometry of the roAp star 10 Aql
Context: We present 31.2 days of nearly continuous MOST photometry of the
roAp star 10Aql. Aims:The goal was to provide an unambiguous frequency
identification for this little studied star, as well as to discuss the detected
frequencies in the context of magnetic models and analyze the influence of the
magnetic field on the pulsation. Methods: Using traditional Fourier analysis
techniques on three independent data reductions, intrinsic frequencies for the
star are identified. Theoretical non-adiabatic axisymmetric modes influenced by
a magnetic field having polar field strengths Bp = 0-5kG were computed to
compare the observations to theory. Results: The high-precision data allow us
to identify three definite intrinsic pulsation frequencies and two other
candidate frequencies with low S/N. Considering the observed spacings, only one
(50.95microHz) is consistent with the main sequence nature of roAp stars. The
comparison with theoretical models yields a best fit for a 1.95Msun model
having solar metallicity, suppressed envelope convection, and homogenous helium
abundance. Furthermore, our analysis confirms the suspected slow rotation of
the star and sets new lower limits to the rotation period (Prot>1 month) and
inclination (i>30\pm10deg.). Conclusions:The observed frequency spectrum is not
rich enough to unambiguously identify a model. On the other hand, the models
hardly represent roAp stars in detail due to the approximations needed to
describe the interactions of the magnetic field with stellar structure and
pulsation. Consequently, errors in the model frequencies needed for the fitting
procedure can only be estimated. Nevertheless, it is encouraging that models
which suppress convection and include solar metallicity, in agreement with
current concepts of roAp stars, fit the observations best.Comment: accepted by A&
MOST photometry of the enigmatic PMS pulsator HD 142666
We present precise photometry of the pulsating Herbig Ae star HD 142666
obtained in two consecutive years with the MOST (Microvariability & Oscilations
of STars) satellite.
Previously, only a single pulsation period was known for HD 142666. The MOST
photometry reveals that HD 142666 is multi-periodic. However, the unique
identification of pulsation frequencies is complicated by the presence of
irregular variability caused by the star's circumstellar dust disk. The two
light curves obtained with MOST in 2006 and 2007 provided data of unprecedented
quality to study the pulsations in HD 142666 and also to monitor the
circumstellar variability.
We attribute 12 frequencies to pulsation. Model fits to the three frequencies
with the highest amplitudes lie well outside the uncertainty box for the star's
position in the HR diagram based on published values.
The models suggest that either (1) the published estimate of the luminosity
of HD 142666, based on a relation between circumstellar disk radius and stellar
luminosity, is too high and/or (2) additional physics such as mass accretion
may be needed in our models to accurately fit both the observed frequencies and
HD 142666's position in the HR diagram.Comment: 10 pages, 11 figures, accepted for publication by Astronomy and
Astrophysic
Gamma Dor and Gamma Dor - Delta Sct Hybrid Stars In The CoRoT LRa01
A systematic search for gamma Dor and gamma Dor - delta Scuti hybrid
pulsators was conducted on the CoRoT LRa01 Exo-archive yielding a total of 418
gamma Dor and 274 hybrid candidates. After an automatic jump correction 194 and
167 respectively, show no more obvious jumps and were investigated in more
detail. For about 25\% of these candidates classification spectra from the
Anglo-Australian Observatory (AAO) are available. The detailed frequency
analysis and a check for combination frequencies together with spectroscopic
information allowed us to identify I) 34 gamma Dor stars which show very
different pulsation spectra where mostly two modes dominate. Furthermore, a
search for regularities in their oscillation spectra allowed to derive
recurrent period spacings for 5 of these gamma Dor stars. II) 25 clear hybrid
pulsators showing frequencies in the gamma Dor and delta Sct domain and are of
A-F spectral type.Comment: Proceedings of the 4th HELAS International Conference held in
Lanzarote, 201
Reduction of time-resolved space-based CCD photometry developed for MOST Fabry Imaging data
The MOST (Microvariability & Oscillations of STars) satellite obtains
ultraprecise photometry from space with high sampling rates and duty cycles.
Astronomical photometry or imaging missions in low Earth orbits, like MOST, are
especially sensitive to scattered light from Earthshine, and all these missions
have a common need to extract target information from voluminous data cubes.
They consist of upwards of hundreds of thousands of two-dimensional CCD frames
(or sub-rasters) containing from hundreds to millions of pixels each, where the
target information, superposed on background and instrumental effects, is
contained only in a subset of pixels (Fabry Images, defocussed images,
mini-spectra). We describe a novel reduction technique for such data cubes:
resolving linear correlations of target and background pixel intensities. This
stepwise multiple linear regression removes only those target variations which
are also detected in the background. The advantage of regression analysis
versus background subtraction is the appropriate scaling, taking into account
that the amount of contamination may differ from pixel to pixel. The
multivariate solution for all pairs of target/background pixels is minimally
invasive of the raw photometry while being very effective in reducing
contamination due to, e.g., stray light. The technique is tested and
demonstrated with both simulated oscillation signals and real MOST photometry.Comment: 16 pages, 23 figure
- …